Study On Metal Powder Flux Cored Wire And Heat Treatment For 25Cr2Ni4MoV Steel

Fans are widely employed in various fields of the national economy,so there is a great demand for the impeller,the core components of the fan.Due to the complicated working conditions of the fan impeller,the size and mechanical properties of the impeller are crucial.As a quenched and tempered high-strength steel,25Cr2Ni4MoV alloy steel needs to be welded and tempered before it can be applied to fan impellers.However,due to the lag in the development of welding materials,25Cr2Ni4MoV alloy high-strength steel lacks advanced welding materials that match its strength and toughness.So far,people still mainly use manual arc welding to produce fan impellers.In order to adapt to the development trend of automated production,it is imminent to develop a welding wire that matches 25Cr2Ni4MoV alloy steel.In order to design flux cored wire that matches the performance of the base material,this work reveal the strengthening and toughening mechanism of 25Cr2Ni4MoV steel by comparing its microstructure,grain boundary,carbides and mechanical properties under different heat treatment conditions.The research indicated show that the microstructure of the supplied 25Cr2Ni4MoV steel is granular bainite containing a large amount of massive M-A islands.This structure contains only a small amount of large-angle grain boundaries,which has a very weak ability to hinder crack propagation.There is no shear lip on the impact fracture surface but there are a large number of cleavage platforms and secondary cracks in the radiation zone.In contrast,the 25Cr2Ni4MoV steel structure after quenching and tempering is complete martensite structure.Further,we have found that with the refinement of martensite blocks,the large-angle grain boundaries increase significantly,and a large number of fine carbides precipitate along the grain boundaries.Therefore,the strength and plastic toughness of steel are significantly improved.To sum up,after 920? oil quenching and 580? tempering treatment,the martensite structure and carbides get the optimal combination,and the comprehensive mechanical properties of steel are better than others.On this basis,the weld metal solidification and phase transformation mechanism of alloy high-strength steel is analyzed.Based on the chemical composition of 25Cr2Ni4MoV steel,the alloying system of the present metal powder-cored wires was selected as Cr-Ni-Mo-Mn-V-Nb-Si-Re,which was formulated to obtain lath martensite.Five wires containing different combinations of alloying elements,especially various combinations of Cr and Ni levels,were used to weld 25Cr2Ni4MoV steel.These flux-cored wires have good welding process performance,fine weld ripples,and no defects such as gas pores and slag inclusions.All the tensile specimens of the welded joint are broken from the weld seam.The results show that when the Cr content is 5.5 wt.%and the Ni content is 19 wt.%,23 wt.%in the wires(3#and 4#),the weld metal structure is mainly finer lath martensite.Their tensile strengths are 920.67MPa and 892.22MPa,and impact energy is 43J and 61J,which are close to the base material.Furthermore,combined with the quenching and tempering process of 920? oil quenching and 580? tempering,the microstructure and mechanic of the welded joints quenching and tempering before and after welding were compared.The results show that the microstructure of the welded joints quenched-tempered before welding is not uniform,and the HAZ microstructure is transformed into coarser lath martensite due to the welding thermal cycle.However,the overall structure of the welded joints quenched-tempered after welding is relatively uniform after welding.The microstructure of HAZ is transformed into fine lath martensite after the quenching and tempering treatment.In addition,all tensile specimens of welded joints were broken from the weld,thus the weld was still the weak part of the welded joint.However,it can be seen that the welded joints quenched-tempered after welding have higher strength.These test results provide a reference for the strengthening and toughening of the weld metal and the welding processing method.